Gram-negative bacteria possess both an inner membrane and an outer membrane. Collectively, the proteins contained in these membranes are referred to as integral membrane proteins. Native integral membrane proteins can be extracted from gram-negative bacteria in relatively small quantities. Recombinant expression techniques permit these proteins to be expressed from bacteria in increased quantities.
Small scale batch purification of such native or recombinant integral membrane proteins has involved an extraction step utilizing centrifugation to extract protein from bacterial cell lysate, followed by downstream purification using conventional techniques.
However, centrifugation is not preferred for extraction of such proteins on a larger scale, because it is a cumbersome process. A larger scale extraction process is desirable in order to obtain quantities of proteins sufficient for economical manufacturing.
Thus, there is a need for a process for extracting native or recombinant gram-negative integral membrane proteins which avoids the use of centrifugation and is therefore more amenable to scale-up. Two such proteins are the lipidated outer membrane proteins P4 and P6 of Haemophilus influenzae. 